The present invention relates generally to power conversion and conditioning and, more particularly, to a compensator for reducing current harmonics from an unbalanced load.
Power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids designed so as to be extremely efficient in delivering large amounts of power. To facilitate efficient distribution, power is delivered over long distances as low frequency three-phase AC current.
Despite being distributable efficiently, low frequency AC current is sometimes not suitable for end use in consuming facilities. Thus, prior to end use, power delivered by a utility has to be converted to a usable form. To this end, a typical power “conditioning” configuration includes an AC-to-DC rectifier that converts the utility AC power to DC across positive and negative DC buses (i.e., across a DC link) and an inverter linked to the DC link that converts the DC power back to three-phase AC power having an end-useable form (e.g., three-phase relatively high frequency AC voltage). A controller controls the inverter in a manner calculated to provide voltage waveforms required by the consuming facility.
Motors and their associated loads are one type of common inductive load employed at many consuming facilities. While the present invention is applicable to different load types, to simplify this explanation, an exemplary motor with an associated load will be assumed. To drive a motor, an inverter includes a plurality of switches that can be controlled to link and delink the positive and negative DC buses to motor supply lines. The linking/delinking sequence causes voltage pulses on the motor supply lines that together define alternating voltage waveforms. When controlled correctly, the waveforms cooperate to generate a rotating magnetic field inside a motor stator core. The magnetic field induces (hence the nomenclature “induction motor”), a field in motor rotor windings. The rotor field is attracted to the rotating stator field, and hence the rotor rotates within the stator core.
Generally, a three-phase voltage source inverter is used to drive a three-phase balanced load. Under this condition, the inverter itself generates only high order current harmonics to its DC link side. The average current flowing inside the DC link side is constant. This can dramatically reduce the ripple current of its DC link capacitor. Thus, it is much easier for the drive to generate high quality input current waveforms using various topologies, (e.g., multiple phase rectifier system, regenerative drive, passive and active filtering rectifier system, etc.) Typically, it is not a problem for a standard designed product to meet IEEE 519 current harmonics specifications under three-phase balanced load conditions.
However, it is not uncommon to use a three-phase inverter product to drive a single-phase load such as a heater. This arrangement reduces the number of drive types that a user must stock and maintain. Due to a single phase configuration, a large number of low order current harmonics are generated in the DC link. With these low order harmonics, a corresponding large number of low order current harmonics can be generated by the rectifier system. It is typically not possible to have a standard design inverter drive a one phase load and still meet IEEE 519 current harmonics specifications. Moreover, a significant amount of the current and voltage stresses are added to either the DC link or the rectifier side components, thus reducing the reliability of the overall drive.
To this end, FIG. 1A illustrates the input current of a three-phase drive system with an 18 pulse front end rectifier driving a single-phase load, and FIG. 1B illustrates the input current of a three-phase drive system driving a conventional three-phase load. Note the distortions in the input current evident in FIG. 1A when a single-phase load is driven. FIG. 2A illustrates the DC link current of a three-phase drive system driving a single-phase load, and FIG. 2B illustrates the DC link current of a three-phase drive system driving a conventional, three-phase load. Again notice the large amount of ripple voltage seen in the DC link current under single-phase load conditions. In addition to causing input current and DC link distortions, the low-order current associated with a single-phase load driven by a three-phase converter can cause stability problems in controlling of the rectifier.